1. Field of the Invention
The present invention relates to a method for forming a packaging material having at least one film layer that interacts with a packaged product and/or that provides a barrier between the interior and the exterior of the package. This interactive/barrier layer, as well as a sealant layer, is formed during an in-line process prior to final orientation of the film.
2. Description of Related Art
Oil-containing foods that are packaged and stored at ambient conditions are subject to limited shelf lives in part due to the fact that the oil therein degrades through oxidative reactions. The shelf life of a packaged product refers to that period of time wherein the qualities of the product are not discernibly different from freshly made product. Over time, moisture changes and oil degradation will adversely affect the aroma, texture, and taste of the packaged food. Such concerns have been addressed in the past by packaging oil-containing foods in packages that contain moisture and oxygen barrier films, opaque films that reduce light-struck reactions, and by flushing the package with an inert gas such as nitrogen. These measures are successful to some degree in extending the shelf life of oil-containing packaged foods. Inevitably some degradation of food product within a package will occur and the measures taken to prevent oxidation, such as the use of barrier films, then act to entrap the undesirable by-products, such as the aldehydes that impart off-flavors associated with rancidity and that contribute to staling of the food product.
Further, recent prior art approaches to extending shelf life not only include ways to reduce the oxidative degradation of oil but they also give consideration to build-up of oxidative by-products within the package such as aldehydes. These approaches utilize a layer in the packaging material that has a desired unique physical packaging property. As defined herein, unique physical packaging property means a property of a packaging material layer that interacts with a packaged food product and/or that provides a barrier of some type between the interior and the exterior of the package. Such barrier may inhibit moisture, oxygen, light and the like from entering or leaving the package. Therefore a film layer having a unique physical packaging property such as scavenger of oil degradation products has been used as a film layer of the package to absorb or scavenge the oil degradation products. Details of films having such unique physical packaging properties are disclosed in PCT application No. PCT/US92/05514 and Publication No. WO 93/01049 and incorporated herein by reference in its entirety. Thus, the package is constructed in such a way that oil degradation by-products build-up within the package is significantly reduced.
A preferred aldehyde-absorbing composition known in the art is polyethyleneimine (PEI). Polyethyleneimine contains primary and secondary amine groups that are highly reactive with aldehydes such as pentanal, hexanal, and heptanal, but significantly less so with other organic compounds such as 2-methylbutanal, toluene, methylpyrazine and 4-heptanone. Polyethyleneimine may be used in direct contact or, preferably, in indirect contact with foods and is stable when disposed within a packaging material. It is known as a primer for laminated film structures that can be used as packaging materials. In U.S. Pat. No. 4,439,493 to Hein, et al., polyethyleneimine is disclosed as a primer for a multilayer oriented heat sealable film structure. This reference is representative of the use of polyethyleneimine in prior art packaging material. Other coating compositions than PEI also provide unique packaging physical property improvements such as moisture barrier, flavor-aroma barrier, oxygen absorption, flavor enhancements and the like. Examples of such coatings, in addition to PEI, include acrylic, PVOH, flavorants, oxygen absorbers, and PVDC.
The difficulty with the prior art arises in attempting to form a packaging material having a layer of film with a desired unique physical packaging property wherein the finished product is relatively inexpensive and also has superior desired properties. Generally, the prior art requires expensive, critical steps in the application of a layer of material having the desired unique physical packaging properties to a base layer such as polypropylene or the prior art uses an out-of-line process to form the various layers of the packaging material and does not always provide a packaging film that will produce the desired function and have the necessary physical properties at reasonable costs.
For example, in U.S. Pat. No. 5,156,904 an in-line process is disclosed to extrude PEI on a base film prior to its being stretched in an orthogonal direction. After being stretched orthogonally, the film made by this process can then be coated in an out-of-line process with an extruded layer of copolymer or bonded to a polymer such as SURLYN.RTM. 1703 produced by I. E. DuPont DeNemours Company of Wilmington, Del. to form a multilayered product. However, the additional steps in an off-line process are expensive. Also, since the film is stretched before applying the extruded layer of copolymer or polymer such as SURLYN.RTM., the extruded layer of copolymer or the SURLYN.RTM. is frequently thicker than necessary. For instance, in food packages, disclosed in PCT/US92/05514 above, the sealant is coated on PEI to separate it from the food product and to allow the PEI to act as a scavenger. The greater the thickness of the sealant, the less desirable the result because of the greater distance the volatile materials must travel to reach the PEI.